The role of the hyporheic zone (HZ; the saturated sediment region below and alongside the streambed) in biogeochemical cycling has been well documented in recent years, resulting in its designation as a river's liver. The streambed and bank sediments comprising the HZ provide diverse redox conditions, contact between solutes and microbes, and long residence times relative to surface water. This dynamic environment has been shown to support many reactions that can attenuate excess nutrients and anthropogenic contaminants from stormwater, recycled water, and non-point sources. These reactions include nitrification and denitrification, carbon cycling, pathogen mitigation, metal immobilization, and attenuation of organic micropollutants. Although the HZ can effectively improve the quality of water along hyporheic flowpaths, its contribution to reach-scale water quality is often transport-limited. In other words, large percentages of flow in natural systems tend to remain in the surface stream, bypassing potential treatment within the HZ. For example, denitrification has been measured within an HZ where small effects on stream nitrate concentrations were observed. In contrast, hyporheic denitrification in another stream has been shown to reduce stream nitrate levels by 10%, demonstrating that the HZ can significantly improve water quality under the right circumstances. Generally, the relevance of the HZ and in-stream biofilms is greatest in shallow, low-discharge streams where surface flow is minimized relative to hyporheic flow. However, even limited hyporheic flow over long distances may be effective at removing pollutants, although solute dynamics beyond the reach scale can be difficult to quantify.
Hyporheic exchange is caused by pressure gradients due to streambed hydraulic conductivity (K) heterogeneities, bedforms, woody debris, channel sinuosity, and topography. In-stream restoration studies have shown that cross-vane structures, bedforms, pool-and-riffle sequences, and woody debris added to channels can induce hyporheic flow.
Few studies have considered modifications to streambed K to improve circulation between surface water and hyporheic sediments. Relatively high K zones cause flow convergence, whereas flow diverges around relatively low K zones. In a streambed setting, these convergent and divergent flows can cause streamwater downwelling and hyporheic water upwelling, respectively.
Disclosed herein are methods and systems for modular streambed K modifications. These modifications are termed Biohydrochemical Enhancements structures for Streamwater Treatment (BEST), and may aid in promoting hyporheic exchanges and corresponding hyporheic residence times, without disrupting surface flow. By not disrupting the surface flow of water in the stream, the BEST module may reduce impact on stream residents, for example fish, and reduce regulatory impact, without altering stream aesthetics. In some embodiments, BEST modules are not visible because they are subsurface, while other restoration practices are visible and alter flow and sediment transport. BEST efficacy is evaluated for several combinations of in situ sediment K and slope, and the results are placed in the context of enhancing removal of various contaminants, for example pathogens and pathogen indicators, organic micropollutants, toxins, metals and the nutrients nitrogen (N) and phosphorous (P) from stream water or water in constructed waterways. Also disclosed is the use of geomedia, which may be natural or synthetic porous media, treated or untreated, reactive or non-reactive, that aids in the removal, modification, inactivation, or metabolism of contaminants. BEST may be used in urban or agricultural channels, where BEST modules comprising a combination of native sediments and reactive porous media may help to remove these and other contaminants derived from storm water runoff, recycled water, irrigation return flow, or natural waters.